1. Field of the Invention
The present invention relates to a magnetic memory device and, in particular, a magnetic random access memory (MRAM).
2. Description of the Related Art
A magnetic random access memory (MRAM) attracts attention as a memory device having high erase/write endurance and a high access speed. A storage element used as the MRAM has a structure in which a magnetic material element such as a magnetic tunnel junction (MTJ) element and a transistor operated as a switch are connected in series with each other, and is called an MRAM cell.
As a data write scheme in the MRAM, a magnetic field writing scheme has been conventionally used. In the magnetic field writing scheme, a magnetic field is applied to the MTJ element to write “0” or “1”. A read operation is performed by indirectly measuring an electric resistance of the MTJ element by a current of the MTJ element or a voltage across both the ends of the MTJ element.
On the other hand, as a data write scheme in the MRAM in recent years, a spin injection write scheme which is advantageous for achieving area reduction has been studied. In the spin injection scheme, “0” or “1” is written by changing a direction of current flowing to a MTJ element.
A method of reading data of a spin injection MRAM is basically the same as that in a magnetic field writing MRAM. More specifically, a current flowing in an MTJ element of an MRAM cell or a voltage across both the ends of the MTJ element is measured, and an electric resistance of the MTJ element is indirectly measured to perform a read operation. In the write operation, a reference cell having a reference MTJ element having a resistance obtained by imitating an intermediate resistance between a high resistance and a low resistance of the MTJ element of the MRAM cell is used. A constant current is caused to flow in the MRAM cell and the reference cell, and a difference voltage generated by a difference between the resistances of the MTJ elements of both the cells is amplified by a sense amplifier. A detection output signal from the sense amplifier is latched by a latch circuit to decide read data (digital value).
As described above, in the spin injection MRAM, in any one of a data write mode and a data read mode, a current is supplied to the MTJ element of the MRAM cell. Therefore, when the read current is large, data is erroneously written, and the stored data is disadvantageously damaged. In particular, in the above example, a current generated when “0” is written in the MTJ element and a current generated when “1” is read from the MTJ element have the same direction. For this reason, when “1” is written in the MTJ element, data may be damaged in a read operation. This phenomenon is called “read disturb”.
In order to avoid the read disturb, a read current may be reduced. However, in this case, a level of the detection output signal decreases. For this reason, a data read rate, i.e., an access rate disadvantageously decreases.
On the other hand, as a method of avoiding the read disturb without decreasing the access rate, a method of shortening a read time as compared to a write time is proposed (JP-A 2007-134027 (KOKAI)). JP-A 2007-134027 (KOKAI) describes that a read current is set at a position having a margin from a magnetization inverted region to make it possible to suppress read disturb.
MRAM cells the number of which is equal to a bit rate of a memory capacity are used. Magnetization inversion characteristics of the MTJ elements vary due to fluctuations in MTJ elements. Therefore, even when a read current is reduced, or even when a read time is made shorter than a write time, read margins of the MTJ elements of all the MRAM cells cannot be easily secured in manufacture. In particular, in a spin injection scheme suitable for a mass-storage volume, since a large number of MTJ elements are used, a yield disadvantageously decreases.